MAINTAINERS: add missing mm_slot.h file THP section

This seems to be the most appropriate place for this file.

[lorenzo.stoakes@oracle.com: also add mm_slot.h to KSM section]
  Link: https://lkml.kernel.org/r/685747e2-a8cb-4620-a0c0-5cd9048d69b8@lucifer.local
Link: https://lkml.kernel.org/r/20250722171904.142306-1-lorenzo.stoakes@oracle.com


Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Acked-by: Zi Yan <ziy@nvidia.com>
Acked-by: Nico Pache <npache@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Dev Jain <dev.jain@arm.com>
Acked-by: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Christoph Lameter (Ampere) <cl@gentwo.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>